A voltage-controlled oscillator having an LC resonator section that uses an inductor L and a variable capacitance element C the capacitance value of which is changed by a control voltage is known in the art. This voltage-controlled oscillator is further provided with an amplifier section for maintaining the oscillation produced by the LC resonator section, and a voltage-controlled oscillator in which the variable capacitance element used in the LC resonator section employs varactor diodes and one in which the LC resonator section employ MOS varactors are available.
Further, a voltage-controlled oscillator in which MOSFETs are used as variable capacitors in addition to varactor diodes is described in Patent Document 1. In the voltage-controlled oscillator described in Patent Document, an LC resonator circuit has an inductance section, a capacitance section and a control voltage terminal, and the capacitance section has serially connected varactor diodes and MOSFETs. The voltage control terminal is connected to the cathodes of the varactor diodes and to the gates of the MOSFETs and the capacitance can be changed more than previously by the control voltage applied to the control voltage terminal.
[Patent Document 1]
    Japanese Patent Kokai Publication No. JP2008-118550A